1. Field of the Invention
Embodiments of the specification generally relate to wireless communications and more particularly to the use of square wave waveforms for testing two-point polar amplifiers.
2. Description of the Related Art
Wireless communications systems generally use radio frequency (RF) signals to transmit data from a transmitter to one or more receivers. Wireless communication systems are frequently used to implement wireless local area networks (LANs) in which data is transmitted and received between computers, servers, Ethernet switches, hubs and the like. A wireless LAN may, for example, enable web page data to be transferred between a server and a computer.
Wireless communication systems often transmit data through transmitters using traditional heterodyne architectures. These architectures typically involve the creation of Cartesian (I and Q) waveforms and then up-mixing the waveforms to a desired frequency. Heterodyne architectures, unfortunately, may require many processing units to handle the Cartesian waveforms, such as a plurality of low-pass filters, baseband amplifiers, mixers and a linear RF amplifier.
Polar transmission architectures may reduce the size and power consumption of a transmitter by, among other things, removing one or more up-mixing stages from the transmitter. Polar transmitters are typically configured to transmit data based upon amplitude and phase waveforms rather than Cartesian I and Q waveforms. One example of a polar transmitter is a two-point polar transmitter in which the phase waveform may be derived from modulating frequency data that is coupled into two frequency inputs. Often, a first modulating frequency input affects the high frequency content of the modulated output and a second modulating frequency input affects the low frequency content of the modulated output of the polar transmitter. A two-point polar transmitter may have relatively greater frequency modulation range than single-point polar transmitters, which may be advantageous when such transmitters are used to transmit relatively greater bandwidth Bluetooth™ waveforms such as the two and three Mbs phase shift keying (PSK) waveforms which may be specified by Bluetooth™ Specification v2.0.
Two-point polar transmitters may be more difficult to test compared to conventional heterodyne transmitters. The fidelity of a transmitted output may depend on the processing of the modulating frequency data within the two-point polar transmitter. Typically, there are separate data processing paths for the first and second modulating frequency inputs. The separate data processing paths may include relatively different processing steps which may lead to a data alignment problem within the two-point polar transmitter. When the data is poorly aligned, the two-point polar transmitter may not optimally transmit the modulated signal.
The data processing within the separate data processing paths may be adjusted in such a manner to compensate for any data alignment issues so that the resulting transmitted signal may have relatively good fidelity (i.e., the transmitted output may include relatively low amounts of distortion). For example, the gain and delay characteristics of the data processing path related to the first modulating frequency input may require some adjustment to more optimally align the processed data with the processed data related to the second modulating frequency input.
One well-known technique for testing the transmitted output of two-point polar transmitters uses a multitone signal. A multitone signal may include two or more simultaneous sinusoidal frequencies. FIG. 1 is a graph of a typical multitone signal 100 that includes three simultaneous frequencies. In one embodiment, the multitone signal may include a first sinusoidal waveform with a frequency F1 and may also include additional sinusoidal waveforms in which the frequency of the additional signals is related to the first sinusoidal signal by an integer. For example, the second sinusoidal waveform may have frequency 2*F1, the third sinusoidal waveform may have frequency 3*F1 and so on. In other embodiments, the multitone signal may include two or more simultaneous sinusoidal frequencies not related by integers. Typically, the amplitudes of each of the simultaneous sinusoids are approximately equal in order to ease the analysis of the FM demodulated output of the two-point polar transmitter. The output of a two-point polar transmitter may be tested by coupling the multitone signal to the two modulating frequency inputs and analyzing the output of the transmitter as is described in greater detail below.
FIG. 2 is a block diagram of a prior art two-point polar transmitter hereinafter referred to as the polar transmitter 200. The polar transmitter 200 includes a polar amplifier 201, a voltage controlled oscillator (VCO) 202, a high frequency processing block 203 a low frequency processing block 204, and a summing node 205. Modulating frequency data is coupled to the polar transmitter 200 through the modulating frequency data input. Within the polar transmitter 200, the modulating frequency is coupled to the high frequency processing block 203 and the low frequency processing block 204. Within the high frequency processing block 203, the modulating frequency data undergoes processing that may affect the high frequency modulated content of the output of the polar transmitter 200. The output of the high frequency processing block 203 is coupled to a first input of the VCO 202. In contrast to the high frequency processing block 203, the modulating frequency data coupled to the low frequency processing block 204 undergoes processing that may affect the low frequency modulated content of the output of the polar transmitter 200. The output of the low frequency processing block 204 is coupled to a first input of the summing node 205 while carrier frequency data is coupled to a second input of the summing node 205. The output of the summing node 205 is coupled to a second input of the VCO 202. The polar amplifier 201 includes frequency and amplitude inputs. The output of the VCO 202 is a modulated sinusoid that is coupled to the frequency input of the polar amplifier 201 while amplitude data is coupled to the amplitude input of the polar amplifier 201. Note that some VCOs may be implemented as closed loop systems, which may be more accurately tested in such a configuration. The output of the polar amplifier 201 is the output of the polar transmitter 200.
Both the high frequency processing block 202 and the low frequency processing block 204 may include gain and delay processing elements that may non-optimally align the processed modulating frequency data coupled to the VCO 202. As described above, the gain and the delay of the high frequency processing block 202 and the low frequency processing block 204 may be adjusted to minimize distortion of the transmitted signal. One method for testing polar transmitters replaces the modulating frequency data with a multitone signal and analyzes the demodulated FM output. In one embodiment, the multitone signal may replace the modulating frequency data through a signal selector (mux) 206 as shown in FIG. 2. Since the polar transmitter frequency modulates each sinusoidal frequency included in the multitone signal, the demodulated output of the polar transmitter 200 should include an output component corresponding to the frequency of each modulated sinusoid. When the amplitudes of the sinusoids included in the multitone signal are substantially the same, the output components of each frequency (FM) demodulated sinusoid from the multitone signal should also have relatively the same amplitude and appear at the correct corresponding frequencies when the processed modulating frequency data is relatively optimally aligned. If the processed modulating frequency data is non-optimally aligned, then the amplitudes of the demodulated sinusoids may vary and, furthermore, the demodulated sinusoids may be displaced from the correct corresponding frequencies (described in greater detail below in the description of FIG. 3).
FIG. 3 is a graph of the FM demodulated output 300 of the polar transmitter modulating a multitone signal. As described above, the multitone signal may replace the modulating frequency data and is selectively coupled to the modulating frequency data input of the polar transmitter. In one example, the multitone signal may include six or more simultaneous sinusoids, however, only the first five are considered herein for illustration purposes. Furthermore, in this example the amplitudes of the sinusoids included in the multitone signal are substantially the same. The first sinusoid has a frequency of F1 Hz. The second sinusoid has a frequency of F2 Hz, the third sinusoid has a frequency of F3 Hz, the fourth sinusoid has a frequency of F4 Hz, and the fifth sinusoid has a frequency of F5 Hz. The output of the polar transmitter is FM demodulated. If the gain and delay of the low and high frequency processing blocks are relatively well aligned, then the amplitude of each FM demodulated component corresponding to a frequency within the multitone signal should be substantially the same because the amplitudes of the sinusoids included in the multitone signal are substantially the same. FIG. 3 shows the FM demodulated output of the polar transmitter with five output components corresponding to five sinusoidal frequencies within the multitone signal. As shown, each output component has approximately the same amplitude and the components appear at frequencies corresponding to the sinusoid frequencies in the multitone signal. If one or more of the output components are higher or lower in amplitude than what may be expected, then the gain or delay of the processing within the high or low frequency blocks may not be optimal. One or more of the output components not appearing at the frequencies corresponding to the sinusoids in the multitone signal may also indicate a sub-optimal gain or delay.
Although the multitone signal may be useful for testing polar transmitters, there are some disadvantages associated with generating and using the multitone signal. Typically, the multitone signal may be generated with a fast Fourier transform (FFT) or a lookup table that is programmed with multitone data values. Both of these approaches, however, need relatively large amounts of die area to implement. Therefore, combining a multitone signal generator with a polar transmitter in a circuit design may result in increased costs due to the additional die area requirements.
Another method for creating the multitone signal uses an external signal generator. This method couples the multitone signal through the modulating frequency data input into the polar transmitter. This approach, however, requires that a number of external pins be available in order to couple with the multitone signal. If the polar transmitter is integrated as part of a larger design, then these external pins may burden the design since they may serve no other purpose other than to support testing. Furthermore, if the modulating frequency data is represented digitally, then a plurality of pins may be required to couple the multitone signal to the polar transmitter. As is well-known, adding pins increases the cost of the integrated circuit. Also, if the overall design including the polar transmitter is relatively small, then the integrated circuit package may not be able to support the additional pins needed to couple the multitone signal.
As the foregoing illustrates, what is needed in the art is a method for testing polar transmitters that requires relatively small amounts of area and relatively fewer pins and provides gain and delay information regarding the low and high frequency processing blocks.